The invention relates to making a multi-perforated part out of ceramic matrix composite material. A particular field of application of the invention is making combustion chamber walls for gas turbines, in particular for airplane jets, which walls are provided with a plurality of perforations enabling them to be cooled by a flow of air.
Ceramic matrix composite (CMC) materials are thermostructural materials, i.e. materials having good mechanical properties and the ability to retain them at high temperature. They comprise fiber reinforcement made of refractory fibers (generally carbon fibers or ceramic fibers) and they are densified by a ceramic matrix or by a combined carbon and ceramic matrix. An interphase layer, e.g. of pyrolytic carbon (PyC) or of boron nitride (BN) can be interposed between the reinforcing fibers and the ceramic matrix in order to improve the mechanical behavior of the material.
Making a part out of CMC normally comprises preparing a fiber preform that is to constitute the reinforcement of the composite material, and densifying the preform with a ceramic matrix, possibly after forming an interphase layer on the fibers of the preform.
The preform is made from one- or two-directional fiber fabrics such as yarns, tows, ribbons, woven cloth, unidirectional sheets, layers of felt, . . . . By way of example, preform shaping comprises steps of winding, weaving, braiding, knitting, or draping plies.
Densification can be performed by a liquid method, i.e. by impregnating the preform with a liquid composition containing a precursor for the ceramic material of the matrix. The precursor is typically a resin which, after being cured, is subjected to heat treatment for ceramization purposes.
Densification can also be performed by a gas method, i.e. by chemical vapor infiltration using a reaction gas containing one or more precursors for the ceramic matrix. The gas diffuses throughout the pores of the fiber preform, and under particular conditions of temperature and pressure it deposits ceramic on the fibers by means of one of the components of the gas decomposing or by means of a reaction taking place between a plurality of components.
The above methods of preparing CMC parts are themselves well known.
Proposals have been made to use CMCs to make the walls of gas turbine combustion chambers. The search for improved efficiency and reduced emissions of pollution require even higher temperature levels in such combustion chambers. That is why proposals have been made to use CMCs to replace the refractory metal alloys that traditionally constitute the walls of combustion chambers. It is still necessary to inject air through the wall, in particular in order to form a cooling film on the wall. Such injection is performed through a large number of small-diameter perforations, typically having a diameter lying in the range 0.2 millimeters (mm) to 0.8 mm.
A method commonly used for making such perforations in metal alloy chamber walls is laser drilling. When applied to CMC walls, that machining technique suffers from the drawback of destroying reinforcing fibers and of laying bare portions of fiber surface or of fiber-matrix interphase surface by locally removing the ceramic matrix. Destroying the fibers weakens the mechanical strength of the material. In addition, bared portions of fiber and interphase surface are exposed directly to the ambient medium, which is oxidizing, thereby making the material sensitive to erosion by oxidation if the fibers are of carbon and/or if they are provided with a PyC or BN interphase.
It might then be envisaged to deposit ceramics on the walls of the perforations, for example in a final step of chemical vapor infiltration after machining. However that affects the calibration of the perforations to a greater or lesser extent, and can even run the risk of clogging them, which reduces the effectiveness with which air is injected through the perforations. In addition, an additional step of chemical vapor infiltration then becomes necessary, and that constitutes a process which is lengthy and expensive.